Place the butter into a large (4 to 6-quart) saucepan over medium-low heat. Once melted, add the onion and a generous pinch of salt and sweat for 2 to 3 minutes. Add the garlic and continue to sweat for an additional 1 to 2 minutes, making certain not to allow onions or garlic to brown.
Add the peas, chicken broth and curry powder. Increase heat to high and bring to a boil. Reduce heat to low, cover and cook at a simmer until the peas are tender and not holding their shape any longer, approximately 45 to 50 minutes. Taste and adjust seasoning as needed. Using care and a stick blender, puree the soup until the desired consistency. Watch out for hot splatters.